The present invention pertains to a method for increasing the degree of absorption during the solid-phase surface hardening of workpieces, especially of geometrically complicated workpieces, by means of laser radiation, in which measures which increase the degree to which the laser radiation is absorbed are taken on the workpiece during the course of the process in the area of the processing zone.
The invention also pertains to an apparatus for increasing the degree of absorption during the solid-phase surface hardening of workpieces, especially of geometrically complicated workpieces, by means of laser radiation, this apparatus being provided with a process gas feed device.
In the processing of materials, e.g., in the surface treatment of structural components, lasers have become well established and are being used in many different areas for a wide range of applications. When laser systems are used, the result of the treatment depends strongly on how much of the laser's power can actually be transferred to the workpiece. The amount of power which can be transferred depends in turn on the degree of absorption at the surface of the workpiece, which itself depends on various factors.
One of the areas of the surface treatment of workpieces is solid-phase hardening with laser radiation. A hardening process of this type is carried out especially on the areas of a workpiece which are subjected, for example, to increased loads or to severe wear or which must withstand elevated temperatures.
By means of the processes used so far for solid-phase hardening of ferrous materials with CO.sub.2 or Nd-YAG laser radiation, the degrees of absorption which can be achieved without additional measures are 10% at a wavelength of .lambda.=10.6 .mu.m and 30% at .lambda.=1.06 .mu.m. The degree of absorption and thus the efficiency of the process, .eta..sub.p P.sub.abs /P.sub.W, where P.sub.abs is the amount of laser power absorbed and P.sub.W is the laser power applied to the workpiece, can be increased only by the use of additional, preparatory work steps. These include the application of absorption-increasing coatings such as graphite or paint to the surface. Because a surface treatment of this type must be carried out in highly defined areas, that is, in the areas which meet the industrial requirements, such absorption-increasing coatings must be applied to the workpiece surface in a highly defined manner with respect to, first, their location and, second, with respect to their uniformity of thickness and homogeneity. For this purpose, spraying or rolling methods can be used, but these involve considerable difficulty when the geometry of the workpiece is complicated. The result is therefore a low degree of reproducibility with respect to the application of the coating, especially with respect to the thickness of the layer and its homogeneity, which leads in turn to unsatisfactory processing results. Reproducibility in this sense, however, is mandatory precisely in the mass production of structural components. In addition, production costs are increased by the additional work step of applying the absorption-increasing coatings before the treatment with laser radiation and by the need to remove the residues of these coatings from the surface after the treatment. To prevent oxidation, nitrogen (N.sub.2) is used as a shielding gas according to the state of the art.